I'm writing a medieval fantasy setting, with a heavy emphasis on the medieval. Most of the "magic" in the setting is "natural magic", which means knowledge of the natural (and often fantastical) properties of animals, minerals, plants, chemical compounds, words, numbers, talismans etc.

I was thinking of using the weird science rules for this kind of magic, tweaked so the "devices" would actually be pre-prepared talismans, potions or other interesting items (so the "light" power is actually a type of rock which, when bathed in clean water, gives off a powerful glow - things like that).

Just wondering if anyone has any thoughts on this type of magic, how this approach might go horribly wrong or anything like that - one problem I am having is in deciding how these powers affect other people - if the raise/lower trait power is a potion, does the wizard give the whole thing, PP and all to another character and they roll "natural magic" to use it? Or does he roll and spend the PP, but the other character can get the benefits whenever they decide to consume the potion?

Firstly, great idea applying weird science to a fantasy setting. As far as your questions, from what I understand, the weird science (or in your case natural magic) skill is not necessarily used for all items. For example, a ray gun would use shooting. I guess it might depend on how you picture the device working. Like, maybe a magical gem that the player throws to create an explosion (the "blast" power) would simply use throwing, whereas something like a talisman of warding ("deflection") would require natural magic. An item such as a rune sword (smite) would require natural magic to activate, then a fighting roll to use. Therefore, certain items would be easier to use by non-magical players than others. As far as the potion, I don't know what kind of roll a player would make for that. Possibly a vigor roll, with each success and raise restoring one wound, and consuming a number of points equal to the "heal" power, with the power points representing how many doses of the potion was left. But then, that would make it much harder for players with low vigor to heal themselves. Perhaps you could just say it restores one wound period and leave it at that.

I wouldn't have the user make any roll to use it. I would allow the alchemist to make the roll when he makes the potion, spending the PP at the same time. The ability to stockpile potions can be offset by the creation time involved. You could also give potions an expiration date, so that the player can't hoard too many. That would be a little more record-keeping though.

There are more polished rules for alchemy and creating magic items in The Fantasy Gear Toolkit. The Pulp GM's Toolkit has more rules for Weird Science including Weird Chemists, which are basically alchemists. It also has an Improved Weird Chemist Edge that lets you create potions that temporarily bestow Edges on the drinker.

I suggest you to take the Shaintar Player Handbook: it has a lot of useful things related to fantasy. Expanded equipment (lot of weapons, armors, shields), some nice additional rules about armor, shields etc.

Of course it has edges and rules about alchemy (that you can use for creating objects that are equivalent of powers, so it's the answer to your natural magic questions) and arcane artificer (that you can use to simulate rune of power carving), potions... Maybe it's the best book to pair to standard S.W. rules, to go deep in fantasy.

Thanks for the ideas guys! I've actually bought the fantasy worldbuilders guide, which has alchemists in, but I think I prefer the weird science approach since it involves a little less book keeping. The only change I'm making is reducing the starting power points of magicians to 5 and increasing their starting powers to 3 - that way they're less "one trick ponies" than starting mad scientists, but can't be using their devices all the time (since each 'device' gets 5 pps).